1. Technical Field of the Invention
The present invention relates generally to cookware as well as the methods for making same and, more particularly, to cookware having a multi-layer, selectively bonded composite construction which provides improved cooking performance and easier cleaning. Another aspect of the invention relates to methods for bonding layers of materials together for making composite blanks of near net shape so as to minimize scrap losses.
2. Description of Related Art
It has long been known to manufacture multi-layer bonded composite cookware in which various materials are joined together to combine the desired physical properties of each of the materials into a composite. For example, the corrosion resistance of stainless steel is ideal for the cooking surface as well as for the exterior surface of cookware; however, the thermal conductivity of stainless steel is not relatively high. On the other hand, aluminum and/or copper offer much higher thermal conductivities and have been bonded to stainless steel to provide well-known composite cookware items such as pots, pans, and the like. Such well-known composite cookware constructions and methods for making are disclosed in U.S. Pat. Nos. 4,004,892; 6,267,830; and 7,820,304, to mention only a few.
The bonded aluminum and/or copper layers, while offering higher thermal conductivity for faster heating, may also result in hot spots on the cooking surface which can cause sticking of the food being cooked and more difficult clean-up of the stainless steel cooking surface. Non-stick surfaces such as PTFE material are also applied to the cook surface to counter the sticking problem; however, the surface underlying the PTFE cooking may also be subject to hot spots which may cause the PTFE to thermally degrade prematurely due to localized hot spots and, thus, limit its useful life. We have heretofore developed a composite construction having at least one bonded core layer of a lower thermal conductivity stainless steel or titanium which acts as a “heat dam” in the composite to cause the heat applied to the cookware to conduct radially and, thus, prevent hot spots from occurring along the cook surface. Thus, the useful life of PTFE (or the like) non-stick surface is increased. See, for example, U.S. Pat. Nos. 6,926,971; 7,278,231; and 7,906,221, all to William A. Groll. It is also known that conventional PTFE non-stick surfaces are prone to scratching if metal tools are used and, therefore, a higher standard of care must be used with such non-stick coatings in order to extend their useful life. Even with careful usage, the PTFE coatings have a finite life as the organic material in the coating tends to naturally degrade and lose its lubricity over time.